Fabrication and Characterization of Ultra-Thin Single-Crystalline Si Fins for Al/Si/Al Josephson Junctions

Josephson junctions (JJs) are key elements to superconducting qubits, providing nonlinear inductance. These JJs typically consist of two aluminum superconductors separated by a thin, amorphous aluminum oxide barrier, which inherently introduces losses due to two-level defects in the amorphous barrier. In an effort to improve the tunnel barrier, we investigate the use of a single-crystal silicon barrier. High aspect ratio, fin-like structures are fabricated from low-loss Si(110) substrates using e-beam lithography and crystallographic selective wet etching following Goswami et al. [1]. This process allows for fins down to 50 nm thick, suitable for low-loss capacitive devices [2]. However, first principle calculations predict that the Si barrier must be less than 10 nm to enable tunnelling [3]. Therefore, we use a repeated digital etch process, involving growing a thin SiO_x layer through controlled oxidation and then etching it. Scanning electron microscopy shows precise control over the etch rate, thinning fins to ~10 nm. After angled Al deposition we form an Al/Si/Al junction and perform IV measurements at ~50 mK to observe Josephson critical currents. Finally, we explore the transition from capacitive to tunneling behavior using flux biased lumped-element resonators and analyze how the resonant frequency depends on the length and thickness of the metallized fin in the presence of a magnetic field.

[1] Appl. Phys. Lett. 121.6 (2022)

[2] arXiv:2408.01369 (2024)

[3] Phys. Rev. B 110.3 (2024): 035302